The corrosion protection of steel substrates is important for many industries, including the automotive and steel industries. Currently, the most common methods of corrosion protection of steel substrates include galvanizing, application of zinc phosphate, electrodeposition of organics, conventional spray or dip priming, oil coating, and combinations thereof. However, for high performance uses in the automotive industry, such methods are associated with the following problems: (1) pollution in the form of volatile organic compounds (VOC), (2) excessive waste disposal, (3) inadequate coverage of recessed areas, and (4) inadequate long term corrosion protection.
It is generally known that a process of organic vapor deposition (OVD), onto a substrate, may be used to provide a thin layer of film, with uniform deposition, no "pin holes," and good edge coverage. Furthermore, such a process does not require the use of solvents, so there is no VOC problem. However, most of the previous work in the area of vapor deposition has been restricted to small objects. In particular, organic vapor deposition has been used for coating micro-electronic or electrical components in electronic and medical devices.
A widely used coating or polymeric material formed by organic vapor deposition is poly-p-xylylene polymer (PARYLENE, commercially available from Union Carbide, Hartford, Conn.). For example, U.S. Pat. No. 3,342,754 discloses vapor deposition of poly-p-xylylene polymers through the cracking of para-xylylene dimers (di-para-xylylene and derivatives) under low pressure. Another patent, JO 1168859A, discloses organic vapor deposition of such a polymer for improving the wear resistance of sliding plastic parts and for imparting corrosion resistance to metal surfaces. However, the latter patent differs from the present process with respect to treatment of the metal surface before and after organic vapor deposition (OVD).
U.S. Pat. No. 4,784,881 discloses organic vapor deposition, using organophosphates as an adhesion promoter to improve adhesion between the deposited film and the substrate. However, this patent does not disclose the use of a metal phosphate, with or without a passivating treatment. Since the patent teaches organic vapor deposition for the purpose of adhesion improvement of substrates used in the electronic industry, it is not believed that such treatment would provide adequate corrosion protection to meet automotive standard requirements. To applicants' knowledge, vapor deposition for corrosion protection of automobiles and structural parts thereof has never been commercially realized.
U.S. Pat. No. 4,950,365 discloses coating poly-p-xylylene over a hard wear-resistant coating such as used in steel tools or instruments. The first step of the two step process disclosed in this patent requires the coating of a metal substrate with a thin, hard layer of a metal compound such as titanium nitride. The second step consists of applying a uniform conformal material exemplified by poly-p-xylylene.
U.S. Pat. Nos. 4,495,889 and 4,518,623 disclose a method and apparatus for organic vapor deposition coating, in which system pressure (related to the film properties) and evaporation temperature (related to the polymerization rate) can be continuously monitored and automatically regulated so that coatings with desired physical properties and deposition rate may be obtained.
Plasma surface modification of a coating formed by organic vapor deposition has also been disclosed. For example, U.S. Pat. No. 4,123,308 discloses a process wherein a low temperature plasma is employed to chemically modify the surface of a poly-p-xylylene film to incorporate oxygen atoms into the backbone of the polymer. Thereafter, a thermosetting. resin such as methyl silicone, methyl phenyl silicone, epoxy, polyurethane, or the like may be chemically bonded to the poly-p-xylylene coated substrate via reaction between oxygen-containing groups at the surface of the poly-p-xylylene and a reactant component of the thermosetting resin.
Notwithstanding the teachings in the prior art, there is a need for a method for providing improved corrosion protection of metal substrates such as employed in automotive production, particularly metal substrates which will thereafter receive one or more coatings to obtain an aesthetically appealing finish. The application of a protective layer or film by organic vapor deposition, for the corrosion protection of metal substrates, must result in good adhesion, good edge coverage, and good barrier properties.